With the exception of the noble metals (Pt, Au, etc.) all metals and their alloys will form oxide films whose removal is required before another metal or metal alloy (hereinafter jointly referred to as "metal") may be electrochemically deposited thereon. Many of these oxides can be substantially removed by appropriate cleaning treatments such as acid etching or sand blasting prior to plating. The oxides formed on metals (e.g., substrate) such as tungsten, copper-tungsten and molybdenum, as well as niobium, titanium and aluminum (referred to here only as examples of readily oxidizable metals) are, however, more complicated and more tenacious than most and require special procedures to ensure proper adhesion of the coating to the base metal.
Several methods have been proposed for the removal of oxide built-up on the surface of readily oxidizable metals. U.S. Pat. No. 2,443,651 to Cannizzaro discloses an electroetching process of tungsten in an aqueous solution containing 5% to 50% hydrofluoric acid while passing an alternating current at a voltage on tile order of 5 volts through the solution. Following the electroetching process, the tungsten article to be electroplated is removed from the acid bath and then immersed in the plating bath and plated.
U.S. Pat. No. 2,835,630 to Alfred, et al. suggests that shot blasting of the tungsten or molybdenum article to be plated is to be preferred over electrochemical cleaning because of the hydrous oxide produced thereby. The shot blasted surface is subsequently coated with copper and then a final metal coating is applied.
Yet another cleaning treatment for readily oxidizable metals is proposed in U.S. Pat. No. 3,699,013 to Miyata et al. The process described therein involves an acid etch, followed by a water rinse which is further followed by a rinse with a dehydrating organic solvent which acts to prevent the build-up of oxides from the oxygen dissolved in the water.
Although the procedures outlined above may produce a more adherent electrodeposited coating on readily oxidizable base metals, they all have the drawback of being comprised of multiple steps carried out with exposure to air or intermediate solutions between the primary steps of cleaning or deoxidizing and electroplating.
Several U.S. Patents suggest processes for cleaning the base metal in the same bath in which the electroplating process occurs. For example, U.S. Pat. No. 923,864 to Levy discloses such a single bath process, however, both the cleaning and plating steps are said to occur at a single current level and simultaneously. In addition, the disclosed electrolyte will not work for the difficult to plate metals.
U.S. Pat. No. 2,546,150 to Brenner discloses a single bath process in which the cleaning is accomplished by application of alternating current (A.C.) prior to application of the direct (D.C.) plating current. The A.C. process described is both an oxidation and a reduction process.
U.S. Pat. No. 4,990,224 to Mahmoud discloses a single bath process wherein the primary deoxidation is the result of a sulfuric acid soak. It is disclosed that the deoxidation may be enhanced by the application of a short period of positive current prior to applying the plating current.
Finally, U.S. Pat. No. 3,627,650 to Seuffert discloses a single bath process for electroplating wherein a potential below the plating potential of the metal to be deposited is applied to the article while in air, followed by immersion of the article into the plating bath and then increasing the potential incrementally to the final plating potential. The application of a potential to the article while in air acts to inhibit dissolution when the article is placed into an aggressive solution.
The above processes, improved as they may be over the usual methods of pre-cleaning and electroplating do not, however, reduce the number of steps in the overall electrodeposition process nor the time required to carry out such overall electrodeposition process. Furthermore, these processes do not make use of Pourbaix diagrams (e.g., knowledge of the thermodynamics) nor do they involve potentiostatically controlled reduction processes.
It has been suggested by Dr. Melvin Goldberg, in a paper titled "Baths for Cleaning and Strike Plating Difficult-to-Plate Metals, Mo, Nb, W, Si and Al, Selected using Pourbaix Diagrams" delivered at the American Electroplaters Society Symposium on Plating on Difficult-to-Plate Metals held during October, 1980 in New Orleans, Louisiana, that Pourbaix diagrams are useful for understanding the chemistry and electrochemistry of cleaning and strike plating solutions.
The disclosures of each of the above mentioned U.S. Patents and the paper of Dr. Goldberg are herein incorporated, in their entirety, by reference.